So none of the connectors that were plugged into the CD changer get connected? That's almost certainly where the problem is. From the wiring diagram, all of the audio outputs actually come from the changer instead of the 32pin connector that goes into the radio. So if my understanding is correct (and I take no responsibility here, haha!), the output frmo the GMOS-06 is going to a connector that is just hanging there doing nothing. Most GM products must go direct-to-amp instead of using a CD changer between? Just a guess.
See the wiring diagrams I posted earlier in the thread for my references, but here goes:
For example (someone should double-check this, seriously), I believe that we need to jumper the connections that normally go into the CD changer. On the 20pin connector, the low-level audio signals go into the changer and right back out to the amplifier. Presumably this is so the changer can inject audio from the XM receiver if so desired, by disabling the input pins (4, 5, 14, 15 on the 20pin connector) from the radio and enabling and splitting the input pins from the XM receiver (5, 12, 14 on the 16pin, using a common neg-signal).
To work around this, we jumper 4 into 6 (left front low positive, tan to tan), 5 into 7 (right front low positive, light green to light green), 14 to 16 (left front low negative, dark green to dark green), 15 to 17 (right front low negative, light green to light green). This allows the audio signal to come out of the GMOS-06 and go "directly" to the amp.
It looks like some level of similar dicking about might be necessary for steering wheel controls, but you said you've got those working so it sounds like you've already tapped into pin 11 (dark blue) on the 16-pin CD changer connector to use with the SWI-RC?